mu Polypeptide phenotypes of lymphoid and myeloid cell lines containing RNA transcripts of the immunoglobulin C mu gene.

We have examined whether polyadenylated-RNA molecules encoded by the immunoglobulin C mu gene (C mu RNA) that have recently been shown to occur in mouse T lymphoma, myeloid tumor, and pre-B lymphoma cell lines are translated. Three T lymphoma, 1 myeloid tumor, and 4 Abelson lymphoma cell lines, all known to contain C mu RNA molecules, were examined for evidence of synthesis of mu polypeptides using radioimmunoassay, biosynthetic peptides were detected in any of the T lymphoma or myeloid cell lines, although mu-chains were readily detected in 2 B lymphoma lines known to contain small amounts of mu mRNA. Detection failure is unlikely to be the result of assay insensitivity and was not caused by lack of an appropriate antiserum reactivity or a requirement for mu-chains to be associated with immunoglobulin light chains. Proteolytic activity that might degrade newly synthesized mu polypeptides in these cell lines was not detected. In 3 of the 4 Abelson lymphoma cell lines containing C mu RNA, corresponding polypeptides were absent, although in the 4th they were easily detected, indicating that transcription of the C mu gene is not the sole determinant of whether a cell will synthesize the corresponding polypeptide. The parallel between the C mu RNA+, mu polypeptide- Abelson cell lines and the T lymphoma cell lines suggests that the onset of C mu transcription may occur in both T and B lineages before translational competence is acquired.